The production of an artificial grain layer on split leather using reactive polyurethane coatings was described for the first time in German Patent Specification No. 878,544. In this process, a solution of an alkyd resin containing free hydroxyl groups and polyisocyanates acting as a crosslinking component is applied to the split leather, and the solvent is then removed by evaporation. According to German Patent Specification No. 872,268 (Example 8), solutions of an NCO-prepolymer and an aromatic diamine are sprayed onto a substrate using a two-jet spray gun, the two solutions only being mixed with one another in the spray jet itself. Similar processes for coating various kinds of substrates are described, for example in German Patent Specification No. 957,564, German Auslegeschrift No. 1,023,449, German Pat. No. 1,240,656, U.S. Pat. No. 3,194,793, German Offenlegungsschrift No. 1,570,524 and U.S. Pat. No. 3,539,424.
The specific coating of leather is comprehensively described, for example, in the Journal "Leder and Hautemarkt", 24 (May, 1972), No. 18, pages 220 to 228, and in Journal of the Society of Leather Technologist and Chemists, 57, pages 59 to 62. As can be seen from these publications, the coating of leather has acquired increasing significance over recent years. When the coated leathers are to be used as starting materials in the manufacture of shoes, the coatings have to meet stringent requirements in terms of flexural strength, scratch resistance, ageing resistance, fastness to light and heat stability. In general, these properties can only be obtained by using polyurethanes.
However, one feature which is common to all previously published processes for coating with reactive polyurethane systems is that either solvents have to be used or, when coating is carried out in the absence of solvents, it is only possible to use certain polyisocyanates and isocyanate prepolymers on account of the excessive viscosity of the reactive system. In reverse coating in particular, considerable importance is attached to reproducing the texture of the temporary substrate as faithfully as possible. This can only be achieved by using low viscosity polyurethane mixtures. However, the use of very low-viscosity (i.e. generally low molecular weight) polyisocyanates introduces the problem that compounds of this kind have a highly toxic effect on account of their relatively high volatility.
In the field of coating textiles with polyurethanes by far the greatest significance is attached to processes in which fully reacted high molecular weight polyurethanes dissolved in suitable solvents (generally dimethyl formamide, optionally in admixture with methyl ethyl ketone or ethyl acetate) are used as starting materials. The solvent is evaporated after the coating solution has been applied to the substrate or alternatively is removed by coagulation in an aqueous coagulation bath. In this particular process, it is possible, by maintaining certain process conditions, to obtain microporous coatings (German Auslegeschrift No. 1,110,607, German Offenlegungsschrifts Nos. 1,444,163; 1,444,165, 1,444,167; 1,694,171; 1,769,277, German Pat. No. 1,270,276). According to German Patent Specification No. 1,694,059, microporous coatings are obtained from solutions of polyurethanes in mixtures of low-boiling solvents and high-boiling non-solvents from which the readily volatile solvent is selectively evaporated after the coating solution has been applied to the substrate.
German Auslegeschrifts Nos. 1,694,180; 1,694,230; 1,694,231 and German Offenlegungsschrift No. 2,034,537 (which corresponds to U.S. Pat. No. 3,788,887) relate to processes for producing microporous films and coatings, in which the starting compounds for polyurethane synthesis (relatively high molecular weight polyhydroxyl compounds, polyisocyanates or isocyanate prepolymers and low molecular weight chain extenders including water) are reacted with one another in an organic solvent which, although dissolving these starting compounds, has increasing difficulty in dissolving the polyurethane formed as the polyaddition reaction progresses, so that, ultimately, it has only a negligible swelling effect upon the fully reactive polyurethane. The still pourable reaction mixture is then applied to a substrate, accompanied by shaping, and polyaddition is completed at a temperature below the softening point of the polyurethane accompanied and/or followed by removal of the solvent. According to another of our own proposals (German Offenlegungsschrift No. 1,694,152 and U.S. Pat. No. 3,625,871), relatively high molecular weight preadducts containing at least two free basic amino groups, optionally dissolved in organic solvents, are dispersed with a non-solvent for the polyadduct to be prepared, optionally using low molecular weight polyamines, the resulting dispersion is mixed with substantially equivalent quantities of a relatively high molecular weight preadduct containing at least two NCO-groups and/or of a low molecular weight monomeric polyisocyanate, the reaction mixture is applied to a substrate, accompanied by shaping, before the end of the polyaddition reaction, polyaddition is completed on the substrate and a microporous coating is obtained following removal of the solvent.
German Offenlegungsschrift No. 1,694,081 (U.S. Pat. No. 3,789,027) describes a process for the production of microporous coatings permeable to water vapor which is distinguished by the fact that monomeric or resin-like polyisocyanates, optionally dissolved in organic solvents, are mixed with a non-solvent (which may even be water) and the resulting mixture is stirred in the absence of emulsifiers to form a dispersion containing isocyanate groups; diamines or polyamines, optionally dissolved in the non-solvent for the polyisocyanates, are then added to the resulting dispersion in quantities substantially equivalent to the isocyanate groups, the reaction mixture is applied to a substrate before the end of the polyaddition reaction, the polyaddition reaction is completed, the solvent used, if any, is removed by evaporation, followed by removal of the non-solvent in the same way. This process is of considerable commercial interest because, basically, it can be carried out in the absence of organic solvents (i.e. solely with water as the liquid reaction medium). Difficulties arise in this process when relatively high molecular weight polyisocyanates of high viscosity are used.
To sum up, therefore, it can be said that, in general, owing to the high viscosity of high molecular weight polyisocyanate conventional processes can only be carried out in the presence of solvents. This not only gives rise to ecological problems, on account of the more or less high toxicity of the solvents, but it also makes the processes uneconomical on account of the high price of solvents. On the other hand, low molecular weight isocyanates, which could be processed in the absence of solvents, can only be used with elaborate safety precautions on account of the toxic properties which they are known to have.